1. Field of the Invention
The method of the present invention relates to the use of extrusion cooling conditions which yield small diameter polybutylene pipe having significantly higher short term (quick burst) strength.
2. Description of the Prior Art
Other processes have been developed and described for the manufacture of polybutylene tubing having certain desired surface smoothness and/or mechanical properties.
U.S. Pat. No. 3,553,303 discusses a process wherein an externally cooled tubing extrudate is cut and immediately immersed in a cooling bath, the hot surface of the pipe's inner diameter being rapidly quenched to provide a smooth surface which is substantially free from rough, undesirable large crystals. These undesireable large crystals or "spherulites" will result from a slow cooling of the inner diameter of the pipe. It should be noted that the object of the '303 invention is the manufacture of thermoplastic tubing having a smooth internal surface, and not necessarily having improved burst pressures.
U.S. Pat. No. 3,686,388 overcomes the prior art cold-forming limitations of elastic memory and/or rupture, by cold-forming the plastic product into a final shape within thirty minutes after the semi-finished product has cooled from the melt, and thereafter storing the shaped product from about 0.5 to 12 hours while avoiding the effects of larger forces on the product. As noted in this '388 patent, cold-formed thermoplastic material exhibits the property of so called elastic memory, wherein the material shows a tendency to return from the shape from which the material was forced during the forming process into the original shape of the material, partially by elasticity, and partially by creep.
In Example 1 of this '388 patent, a solid rod of poly-l-butene having a diameter of 6 mm, a solution viscosity of 4.8, and a melt index of I.sup.5 equal to 0.5 gms per 10 minutes, is continuously extruded in the form of a melt, formed into a straight rod by means of a gauge while still in a pasty condition and cooled with water to room temperature. The temperature of the rod is 22.degree. C. One minute and ten seconds after extrusion, a cut to length piece of rod 1/2 meter long is coiled around a mandrel so that the individual turns are in contact with one another, forming a helically wound rod. After termination of the coiling process which lasts about 11/2 minutes, the mandrel is removed from the now coiled rod. The helically wound rod is then stored for 8 hours without placing any strains thereupon. The shaped article so formed thereafter has sufficient stability for use as a helical spring adapted to apply a light tension load thereto.
It should be noted that this patent also does not teach a method of improving the burst pressure of polybutylene pipe.
In U.S. Pat. No. 3,822,333 a process of forming polyolefin forms having roughened surfaces is disclosed, wherein the melt is drawn through a solvent bath prior to solidification. This reference relates to the use of a shortened solvent bath residence time of 1 minute or less. The time necessary for the surface of the resin in a molten state to be cooled and solidified is very short, in the order of several tenths of a second or at most several seconds. Cooling of the entire shaped article is also usually completed in one minute or less when the article has a thickness of 1 mm or less; and in about 1/2 second when it has a thickness of about 0.03 mm.
U.S. Pat. No. 3,936,524 discloses a process wherein polyisobutylene oxide polymer moldings containing 70 wt % or more of isobutylene oxide, are prepared by heating the polymer to a temperature about 170.degree. C. and then quickly cooling the same by contacting the polymer with a coolant to obtain crystalline resin having a diffraction peak at about 2.tau.=12.degree. in the X-ray diffraction pattern of the resin.
Great Britain Patent 819,396 discloses a process for providing hollow elongated shapes such as polyethylene tubing and pipe having superior stress resistance, wherein the pipe is extruded from a melt of the polyethylene polymer, quenched to a shaped stable form, and wherein thereafter the external surface of the shape is heated to a temperature and for a time sufficient to relax or "anneal" the polymer surface. The thermal treatment after quenching is of sufficient intensity to heat the exterior surface of a shape to a temperature above the melting point of the polyethylene.
And finally, U.S. Pat. No. 4,783,301 discloses a process wherein polybutylene-1 polymer is introduced into a form which defines the shape of the desired part and is then subjected to shear at a temperature from just above to just below the melting point of the polybutene-1 polymer, achieved by positioning the polymer blend between a shell and a mandrel and rotating the two relative to one another. The foregoing "shear" method substantially improves the mechanical properties of the polybutene-1 pipe with a claimed increase of 1500% or more.
A process needs to be developed that improves the burst pressure properties of polybutylene pipe, without the necessity of the complex shear apparatus of the '301 patent, or the post-extrusion annealing process of Great Britian Patent 819,316.